[Ext4] 05 disk Layout

In linux, any file has an inode corresponding to it. That is to say, in a file system, a file has a unique ino to mark it, in the ext4 system, how does ino determine?

When we create a new file or directory, the ext4_create function is called. The call path is ext4_create-> ext4_new_inode. The inode-> ino is defined in the ext4_new_inode function.

Analysis Function ext4_new_inode

Struct inode * _ ext4_new_inode (handle_t * handle, struct inode * dir,

Umode_t mode, const struct qstr * qstr,

_ U32 goal, uid_t * owner, int handle_type,

Unsigned int line_no, int nblocks)

{

...

For (I = 0; I <ngroups; I ++, ino = 0 ){

Err =-EIO;

 

Gdp = ext4_get_group_desc (sb, group, & group_desc_bh );

If (! Gdp)

Goto out;

 

/* 1. check whether there is free indode before loading inode bitmap */

If (ext4_free_inodes_count (sb, gdp) = 0 ){

If (++ group = ngroups)

Group = 0;

Continue;

}

...

/* 1. check whether there is free indode before loading inode bitmap */

Ino = ext4_find_next_zero_bit (unsigned long *)

Inode_bitmap_bh-> B _data,

EXT4_INODES_PER_GROUP (sb), ino );

 

 

Struct inode * _ ext4_new_inode (handle_t * handle, struct inode * dir,

Umode_t mode, const struct qstr * qstr,

_ U32 goal, uid_t * owner, int handle_type,

Unsigned int line_no, int nblocks)

{

...

For (I = 0; I <ngroups; I ++, ino = 0 ){

Err =-EIO;

 

Gdp = ext4_get_group_desc (sb, group, & group_desc_bh );

If (! Gdp)

Goto out;

 

/* 1. check whether there is free indode before loading inode bitmap */

If (ext4_free_inodes_count (sb, gdp) = 0 ){

If (++ group = ngroups)

Group = 0;

Continue;

}

...

Inode_bitmap_bh = ext4_read_inode_bitmap (sb, group );

...

/* 2. In the inode bitmap, find the next 0 bit starting with ino + 1 */

Ino = ext4_find_next_zero_bit (unsigned long *)

Inode_bitmap_bh-> B _data,

EXT4_INODES_PER_GROUP (sb), ino );

/* 3. Check whether free inode is valid: (1) greater than inode_per_group, (2) smaller than first_ino */

If (ino> = EXT4_INODES_PER_GROUP (sb ))

Goto next_group;

If (group = 0 & (ino + 1) <EXT4_FIRST_INO (sb )){

Ext4_error (sb, "reserved inode found cleared -"

"Inode = % lu", ino + 1 );

Continue;

}

...

/* 4. Update inode bitmap. If inode bitmap update fails, search again */

Ext4_lock_group (sb, group );

Ret2 = ext4_test_and_set_bit (ino, inode_bitmap_bh-> B _data );

Ext4_unlock_group (sb, group );

Ino ++;/* the inode bitmap is zero-based */

If (! Ret2)

Goto got;/* we grabbed the inode! */

Next_inode:

If (ino <EXT4_INODES_PER_GROUP (sb ))

Goto repeat_in_this_group;

Next_group:

If (++ group = ngroups)

Group = 0;

}

...

/* 5. Update the free_inodes_count variable in the group */

Ext4_free_inodes_set (sb, gdp, ext4_free_inodes_count (sb, gdp)-1 );

If (S_ISDIR (mode )){

Ext4_used_dirs_set (sb, gdp, ext4_used_dirs_count (sb, gdp) + 1 );

If (sbi-> s_log_groups_per_flex ){

Ext4_group_t f = ext4_flex_group (sbi, group );

 

Atomic_inc (& sbi-> s_flex_groups [f]. used_dirs );

}

}

If (ext4_has_group_desc_csum (sb )){

Ext4_inode_bitmap_csum_set (sb, group, gdp, inode_bitmap_bh,

EXT4_INODES_PER_GROUP (sb)/8 );

Ext4_group_desc_csum_set (sb, group, gdp );

}

Ext4_unlock_group (sb, group );

...

/* 6. Get inode-> ino */

Inode-> I _ino = ino + group * EXT4_INODES_PER_GROUP (sb );

...

}

Steps:

1. check whether there is free indode before loading inode bitmap.

2. In the inode bitmap, find the next 0 bit starting with ino + 1.

3. Check whether free inode is valid: (1) greater than inode_per_group, (2) smaller than first_ino

4. Update inode bitmap. update failed and search again

5. Update the free_inodes_count variable in the group.

6. Get inode-> ino.

 

Therefore, you can use inode-> ino to calculate the group and block of inode:

Group_id

= (Inode-> ino-1)/inodes_per_group

Inode_blkid

= First_inode_blk_in_group

+ (Inode> ino-1) % inodes_per_group)/(inodes_per_block );

Inode_offset

= First_inode_blk_in_group * blk_size

+ (Inode> ino-1) % inodes_per_group) * inode_size

Where:

The reason inode-> ino-1 is that ext4 system does not have file index 0

(1) inode-> ino: it can be obtained through stat file_name;

(2) inodes_per_group: the number of inode in each group. For details, see debugfs;

(3) first_inode_blk_in_group: indicates the start block id of the inode table, which can be obtained by debugfs;

(4) blk_size: Logical Block Size, which can be obtained by debugfs;

(5) inode_size: the size of the space required for an inode to be stored on the disk, which can be obtained by debugfs;

(6) inodes_per_block: the number of inodes that each block can store. You can use blk_size/inode_size to obtain inode_block;

Root @ ubuntu:/mnt/ext4 # debugfs/dev/sdc

Debugfs 1.42 (29-Nov-2011)

Debugfs: stats

Filesystem volume name: <none>

Last mounted on:/mnt/ext4

Filesystem UUID: 883c6632-e61d-4420-b309-0695f321cf9d

Filesystem magic number: 0xEF53

...

Inode count: 327680

...

Fragments per group: 32768

Inodes per group: 8192

Inode blocks per group: 512

...

First inode: 11

Inode size: 256

Required extra isize: 28

...

Group 0: block bitmap at 321, inode bitmap at 337,Inode tableAt 353

24217 free blocks, 8180 free inodes, 2 used directories, 8180 unused inodes

[Checksum 0x25aa]

Group 1: block bitmap at 322, inode bitmap at 338, inode table at 865

31423 free blocks, 8192 free inodes, 0 used directories, 8192 unused inodes

[Inode not init, Checksum 0x94f1]

Group 2: block bitmap at 323, inode bitmap at 339, inode table at 1377

32768 free blocks, 8192 free inodes, 0 used directories, 8192 unused inodes